Unlike unions, they say, Big Business advocates aren’t flooding Capitol phone lines. They’re not winning over skeptical Republicans. And they haven’t made much headway with business-friendly Democrats who are considering voting for the package, either.

That threatens to create a dangerous reality for supporters of a sweeping trade deal with Pacific Rim nations: that it will become more politically tenable for Republicans to be against trade promotion authority legislation than for it.

The chorus of GOP complaints — striking considering the typically close ties between Republican leadership and Big Business — is coming from all over the Capitol. But it’s loudest on the House side.

David Stewart, a top aide to Speaker John Boehner, voiced the frustration of Boehner’s office during a meeting Friday with officials from business lobby groups, telling them their effort is falling short. During the meeting at the offices of the Business Roundtable, Stewart said unions are outworking the business groups on calls to GOP lawmakers’ offices.

“The lobbying effort on the Hill has been abysmal,” one senior GOP aide said. “Calls and letters into member offices are running 10 to 1 against TPA. This is an uphill fight already given the lack of trust in the president and the general unpopularity of TPA, and the current lobbying effort has not made it any easier. If TPA passes in the House it will be despite the downtown coalition and the president, not because of them.”

Of course, trade politics are tricky — the debate over TPA has triggered concerns ranging from job losses in individual House districts to currency manipulation — and it’s not just the business community that’s struggling. President Barack Obama hasn’t yet been able to build enough Democratic support to get the fast-track bill across the finish line. He faces a big obstacle in House Minority Leader Nancy Pelosi (D-Calif.), who is vehemently opposed to the legislation as it’s currently written.

But the unions’ decades-old network is outfoxing groups like the Business Roundtable and the U.S. Chamber of Commerce, critics say. After a brief setback last week, the bill is expected to clear the Senate late this week or perhaps after the Memorial Day recess. Democrats are looking to stall final passage in the hopes of giving opponents more time to mobilize for the closer fight in the House, where the legislation currently lacks the votes to pass.

Another GOP aide said the ratio of opposing calls to those in favor of the trade agreement is even more worrisome, 25 to 1 or worse.

“No one is happy with what they’re doing on trade,” the staffer complained. “The business community is doing a great job of sending trade association letters to the Hill, but they aren’t doing much out in the country. I think they think they’re killing it, which just shows you how bad they are.”

Stewart’s call for the business community to engage more aggressively comes months after the Business Roundtable, the National Association of Manufacturers, the Chamber and others formed a coalition called Trade Benefits America to push fast-track legislation through Congress. The business alliance has said it’s organized hundreds of meetings with lawmakers, spurred tens of thousands of letters and calls and spent more than $1 million on TV ads in 26 markets on broadcast and cable, in addition to Internet, radio and print ads in 30 states.

The coalition’s spokesman, Kevin Madden, described the meeting with Boehner’s aide as a “very constructive conversation.”

“We regularly hear that the advocacy efforts of employers and members of the business community are appreciated, but we are also reminded that every ounce of those efforts is needed in order to combat the opponents of trade who are very active,” Madden said. “We recognize that the anti-trade organizations are dug in on this issue. Quite frankly, they’ve had a 20-year head start.”

Madden said the business community needs to be “unrelenting and keep improving our efforts.”

TPA is a critical component of Obama’s economic agenda. The legislation would empower the president to cut free trade deals that Congress could approve or reject on an up-or-down vote but not amend. Some House Republicans are concerned with the perception that TPA would hand the White House too much additional power. Democrats are mostly concerned about the effects of free trade on American workers.

Trade supporters acknowledge the uphill battle in the House but believe they’re building support. Rep. Paul Ryan (R-Wis.) and other lawmakers supportive of trade have been walking through the policy details with lawmakers.

It’s a dangerous development for those hoping for passage of TPA sometime this summer. House Republicans estimate that 170 to 180 of their members will support the legislation. GOP leadership says roughly three dozen Democrats will need to support the bill to ensure passage.

The timing of votes on TPA in the House is up in the air. The Senate is moving toward approving the measure possibly next week. The House is out of session during the last week of May, and there is some hope that TPA could come to the floor sometime in June.

“We all have the same goal and appreciate the work everyone is putting into this effort,” Boehner spokesman Kevin Smith said. “Everyone knows this is a tough issue but we’re working in a collaborative way and confident we can get this done.”